Vehicle-Mounted Optical Communication System and Vehicle-Mounted Optical Transmitter

ABSTRACT

A vehicle-mounted optical communication system, which uses an optical signal to perform data transmission, comprises a first optical transmitter and an optical receiver. The first optical transmitter, which is mounted on a vehicle, has a multiple quantum well structure, in which an active layer has a quantum well layer of In x Ga 1-x As (where 0.15≦x≦0.35), and includes a first surface emitting laser the oscillation wavelength of which is between 1000 nm and 1100 nm inclusive. The first optical transmitter transmits an optical signal generated by the first surface emitting laser. The optical receiver, which is mounted on the vehicle and connected to the first optical transmitter via a first optical transmission path, receives the optical signal, which was transmitted by the first optical transmitter, via the first optical transmission path.

TECHNICAL FIELD

The present invention relates to a vehicle-mounted optical communicationsystem and an optical transmitter therefor.

BACKGROUND ART

Heretofore, many vehicles have incorporated various devices includingaudio devices, navigation systems, and cameras for capturing imagesoutside of the vehicle. When such many devices are incorporated in avehicle, the vehicle has a vehicle-mounted optical communication systeminterconnecting the devices with optical transmission paths. In thevehicle-mounted optical communication system, electric signals to betransmitted are converted by light source devices having LEDs or thelike into optical signals to pass through the optical transmissionpaths.

The transmission rate on the vehicle-mounted optical communicationsystem is in the range from about 25 to 50 Mbps at present. However, asthere are demands for transmission rate increases, the transmission rateis considered to approach 1 Gbps in the future.

To meet such demands for increased transmission rates, efforts to usesurface-emitting lasers instead of LEDs are being made on research anddevelopment levels (see JP-A No. 2005-26770).

DISCLOSURE OF THE INVENTION

Vehicle-mounted optical communication systems installed in vehicles areused in severe environments. Specifically, the vehicle-mounted opticalcommunication systems are required to be durable in environments attemperatures as high as about 125° C. However, surface-emitting lasershaving an oscillation wavelength band of 850 nm which have heretoforebeen used in vehicle-mounted optical communication systems are of poordurability in environments at temperatures as high as about 125° C.

Consequently, it has been difficult to increase the reliability ofvehicle-mounted optical communication systems which incorporatesurface-emitting lasers having an oscillation wavelength band of 850 nm.

It is an object of the present invention to provide a vehicle-mountedoptical communication system which is highly reliable.

To achieve the above object, a vehicle-mounted optical communicationsystem according to the present invention, adapted to be mounted on avehicle, for performing data transmission with optical signals,comprises a first optical transmitter and an optical receiver.

The first optical transmitter includes a first surface-emitting laserhaving an active layer of a multiple quantum well structure having aquantum well layer of In_(x)Ga_(1-x)As (0.15≦x≦0.35), the firstsurface-emitting laser having an oscillation wavelength ranging from1000 nm to 1100 nm inclusive.

The first optical transmitter and the optical receiver are adapted to bemounted on the vehicle and are connected to each other by a firstoptical transfer path. The first optical transmitter transmits anoptical signal generated by the first surface-emitting laser. Theoptical receiver receives the optical signal transmitted from the firstoptical transmitter through the first optical transfer path.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between the oscillationwavelengths of surface-emitting lasers and the reliability thereof;

FIG. 2 is a diagram showing a vehicle-mounted optical communicationsystem according to a first exemplary embodiment of the presentinvention;

FIG. 3 is a cross-sectional view of a surface-emitting laser;

FIG. 4 is a perspective view of a package;

FIG. 5 is a graph showing the temperature dependency of a modulatingoperation of a surface-emitting laser incorporated in thevehicle-mounted optical communication system according to the firstexemplary embodiment;

FIG. 6 is a diagram showing a vehicle-mounted optical communicationsystem according to a second exemplary embodiment of the presentinvention;

FIG. 7 is a view of a photodetector of the vehicle-mounted opticalcommunication system according to the second exemplary embodiment;

FIG. 8 is a view of a surface-emitting laser incorporated in thevehicle-mounted optical communication system according to the secondexemplary embodiment;

FIG. 9 is a diagram showing a vehicle-mounted optical communicationsystem according to a third exemplary embodiment of the presentinvention; and

FIG. 10 is a view of a photodetector of the vehicle-mounted opticalcommunication system according to the third exemplary embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention will be described indetail below with reference to the drawings.

The inventors of the present invention have considered that thedurability of a surface-emitting laser in severe environments is loweredby the growth of crystalline defects in an active layer due to atemperature rise.

The inventors have considered that it is important to suppress thegrowth of crystalline defects in the active layer of thesurface-emitting laser in order to provide a vehicle-mounted opticalcommunication system which is highly reliable in severe environments.

It has been realized that crystalline defects are easy to grow insurface-emitting lasers having an oscillation wavelength band of 850 nmaccording to the background art because the active layer thereofgenerally comprises a quantum well layer of GaAs and a barrier layer ofAlGaAs. In particular, if the environmental temperature rises or thecurrent density rises, then crystalline defects grow significantly, andthe service life of surface-emitting lasers decreases significantly.

The inventors have studied the relationship between the growth ofcrystalline defects and the oscillation wavelength (the proportion of Inin an In_(x)Ga_(1-x)As layer as a quantum well layer), and reached theconclusion shown in FIG. 1.

The inventors have found that as shown in FIG. 1, the reliability of asurface-emitting laser is greatly improved by increasing the proportionof In into the range of 0.15≦x≦0.35 and keeping the oscillationwavelength in the range from 1000 nm to 1100 nm inclusive. FIG. 1 showssimulated times required until the intensity of light drops 20% when acurrent of given value passes through surface-emitting lasers havingrespective proportions of in (oscillation wavelengths). The reliabilityrepresented by the vertical axis shows relative values of the timesrequired until the intensity of light drops 20%. In FIG. 1, the valuesof reliability represented by the vertical axis are required to be 48 orgreater. The proportion of In in the quantum well layer of thesurface-emitting layer having an oscillation wavelength band of 850 nmis nil.

In an oscillation wavelength range from 1000 nm to 1100 nm inclusive,the added In is considered to pin and reduce the growth of crystallinedefects. In addition, the high differential gain of InGaAs itself isconsidered to greatly reduce the amount of drive current for thesurface-emitting laser, thereby preventing the temperature in the activelayer of the surface-emitting laser from rising for greatly increasedreliability.

FIG. 1 is plotted when the environmental temperature is 100° C. and thetransmission rate is 1 Gbps. It is known that the same results areobtained when the environmental temperature is 125° C.

Vehicle-mounted optical communication systems according to exemplaryembodiments of the present invention comprise an optical transmitterhaving a light source device, a transfer medium for transferring lightfrom the optical transmitter, and an optical receiver for receiving thelight transferred by the transfer medium. The vehicle-mounted opticalcommunication systems serve to transfer data at a high rate of 1 Gbps orhigher. The light source device comprises a surface-emitting layerhaving a GaAs substrate and an active layer disposed on the GaAssubstrate. The active layer is of a multiple quantum well structurehaving a quantum well layer of In_(x)Ga_(1-x)As (0.15≦x≦0.35), and thesurface-emitting laser has an oscillation wavelength ranging from 1000nm to 1100 nm inclusive.

With the above arrangement, the active layer of the surface-emittinglaser has a quantum well layer of In_(x)Ga_(1-x)As (0.15≦x≦0.35) and thesurface-emitting laser has an oscillation wavelength ranging from 1000nm to 1100 nm inclusive. The surface-emitting laser is highly reliablein severe temperature environments. The vehicle-mounted opticalcommunication system which incorporates the surface-emitting laser ishighly reliable.

There is known a surface-emitting laser with an active layer having aquantum well layer of In_(0.2)Ga_(0.8)As as disclosed in JP-A No.10-233559. However, it has heretofore not been known at all to increasedurability against environmental temperatures for high reliability insevere temperature environments by keeping the proportion of In in aquantum well layer of In_(x)Ga_(1-x)As in the range of 0.15≦x≦0.35 andkeeping the oscillation wavelength in the range from 1000 nm to 1100 nminclusive. In other words, it has heretofore not been anticipated to beable to provide a highly reliable vehicle-mounted optical communicationsystem by incorporating therein a surface-emitting laser having aquantum well layer of In_(x)Ga_(1-x)As (0.15≦x≦0.35) and an oscillationwavelength ranging from 1000 nm to 1100 nm inclusive.

Exemplary embodiments of the present invention will be described indetail below with reference to the drawings. Similar components aredenoted by similar reference characters throughout views and will not bedescribed below.

1st Exemplary Embodiment

FIG. 2 shows vehicle-mounted optical communication system 1 according tothe present invention.

Vehicle-mounted optical communication system 1 comprises opticaltransmitter 11 having light source device 14, transfer medium 12 fortransferring light from optical transmitter 11, and optical receiver 13for receiving the light transferred by transfer medium 12. Thevehicle-mounted optical communication system serves to transfer data ata high rate of 1 Gbps or higher.

As shown in FIG. 3, light source device 14 comprises surface-emittinglaser 15 including GaAs substrate 151 and active layer 154 disposed onGaAs substrate 151. Active layer 154 is of a multiple quantum wellstructure having a quantum well layer of In_(x)Ga_(1-x)As (0.15≦x≦0.35).Surface-emitting laser 15 has an oscillation wavelength ranging from1000 nm to 1100 nm inclusive.

Details of vehicle-mounted optical communication system 1 will bedescried in detail below.

Optical transmitter 11 is incorporated in a camera for capturing imagesoutside of the vehicle. An electric signal based on an image captured bythe camera is supplied to optical transmitter 11.

Optical transmitter 11 comprises light source device 14 and drivecircuit 16 for energizing light source device 14. Drive circuit 16 issupplied with an electric signal based on an image captured by thecamera, and modulates light emitted by surface-emitting laser 15 oflight source device 14 with the electric signal.

As shown in FIG. 3, surface-emitting laser 15 comprises GaAs substrate151 as a semiconductor substrate, first DBR (Distributed BraggReflector) layer 152 disposed on GaAs substrate 151, cladding layer 153disposed on first DBR layer 152, active layer 154 disposed on claddinglayer 153, second cladding layer 155 disposed on active layer 154,current constricting layer 156 disposed on second cladding layer 155,and second DBR (Distributed Bragg Reflector) layer 157 disposed oncurrent constricting layer 156.

Surface-emitting laser 15 is of the vertical resonator type.

First DBR layer 152 is an n-type semiconductor multilayer filmcomprising an alternate stack of n-type AlGaAs films and n-type GaAsfilms.

Cladding layer 153 comprises a GaAs layer, for example.

Active layer 154 is an MQW (Multiple Quantum Well) layer comprising analternate stack of quantum well layers of In_(x)Ga_(1-x)As (0.15≦x≦0.35)and GaAs barrier layers. In the present exemplary embodiment, thequantum well layers comprise an In_(0.25)Ga_(0.75)As layer, andsurface-emitting laser 15 has an oscillation wavelength of 1070 nm.

Second cladding layer 155 comprises a GaAs layer, for example.

Current constricting layer 156 comprises an AlAs layer. Currentconstricting layer 156 includes low-resistance region 156A.Low-resistance region 156A is sandwiched between high-resistance regions(oxide regions) 156B having a higher resistance value thanlow-resistance region 156A and formed by a steam oxidation process.

Second DBR layer 157 is a p-type semiconductor multilayer filmcomprising an alternate stack of p-type AlGaAs films and p-type GaAsfilms.

Upper electrode 158 is disposed on second DBR layer 157, and lowerelectrode 159 is disposed on first DBR layer 152.

Surface-emitting laser 15 is produced by successively growing layers 152through 157 on GaAs substrate 151 by MOVPE (Metal-Organic Vapor PhaseEpitaxy), gas source MBE (Molecular Beam Epitaxy), or the like.

Surface-emitting laser 15 of light source device 14 and drive circuit 16are sealed in package 17 as shown in FIG. 14. Package 17 is made ofplastics or metal and is of a hollow cylindrical shape. Surface-emittinglaser 15 and drive circuit 16 are fixedly mounted on the bottom surfaceof package 17. Package 17 is filled with an electrically insulativeliquid or gel (e.g., silicone-base liquid or gel).

Vehicle-mounted optical communication system 1 will be described belowwith reference to FIG. 2 again.

Transfer medium 12 serves to transfer an optical signal fromsurface-emitting laser 15 of light source device 14, and comprises, forexample, an optical fiber such as a polymer-clad optical fiber (PCF) orthe like.

Optical receiver 13 serves to receive the optical signal from transfermedium 12, and comprises photodetector 131, amplifying circuit 132, andcode generating circuit 133.

Photodetector 131 may be any photodetector insofar as it can detectlight in a wavelength range from 1000 nm to 1100 nm, emitted fromsurface-emitting laser 15.

Photodetector 131 converts the optical signal into an electric signal,which is decoded as it is processed by amplifying circuit 132 and codegenerating circuit 133.

Optical receiver 13 is incorporated in a monitor or the like installedin the vehicle, for example. The electric signal decoded by opticalreceiver 13 is transmitted to the monitor or the like installed in thevehicle.

When the transfer medium 12 of vehicle-mounted optical communicationsystem 1 was in the form of a polymer-clad optical fiber (PCF) having alength of 10 m, the transmission rate was 1 Gbps at an environmentaltemperature ranging from −40° C. to 125° C. At an environmentaltemperature of 100° C., vehicle-mounted optical communication system 1was reliable for 5000 hours (it took 5000 hours before the intensity oflight dropped 20%).

An experiment was conducted about the temperature dependency of amodulating operation of surface-emitting laser 15 incorporated invehicle-mounted optical communication system 1.

The results are shown in FIG. 5. It can be confirmed thatsurface-emitting laser 15 could operate at a high rate of 4 Gbps at ahigh temperature of 150° C.

According to the present exemplary embodiment, active layer 154 ofsurface-emitting laser 15 includes a quantum well layer ofIn_(x)Ga_(1-x)As (0.15≦x≦0.35), and surface-emitting laser 15 has anoscillation wavelength ranging from 1000 nm to 1100 nm inclusive.Surface-emitting laser 15 thus constructed is highly reliable in severetemperature environments. Vehicle-mounted optical communication system 1which incorporates surface-emitting laser 15 is thus highly reliable.

According to the present exemplary embodiment, surface-emitting laser 15and drive circuit 16 are accommodated in package 17, and package 17 iswith an electrically insulative liquid or gel. The liquid or gel inpackage 17 is capable of absorbing vibrations in the vehicle.

According to the present exemplary embodiment, the liquid or gel inpackage 17 comprises a silicone-base liquid or gel. Since thesilicone-base liquid or gel is of excellent thermal conductivity, it canquickly dissipate the heat generated by active layer 154 ofsurface-emitting laser 15, thereby reliably preventing surface-emittinglaser 15 from being deteriorated.

Inasmuch as the silicone-base liquid or gel has a refractive indexgreater than 1, the light emitted from surface-emitting laser 15 travelsstraight in package 17 without being spread, reducing any coupling lossbetween package 17 and the optical fiber of transfer medium 12.Specifically, the silicone-base liquid or gel in package 17 reduces thecoupling loss from 2 dB, which occurs if package 17 is dispensed with,to 1 dB.

2nd Exemplary Embodiment

A second exemplary embodiment of the present invention will be describedbelow with reference to FIG. 6.

Vehicle-mounted optical communication system 2 according to the presentexemplary embodiment comprises optical transmitter 11 having lightsource device 14 similar to the light source device according to thepreceding exemplary embodiment, transfer medium 12 for transferringlight from optical transmitter 11, and optical receiver 23 for receivingthe light transferred by transfer medium 12. Vehicle-mounted opticalcommunication system 2 also includes second optical transmitter 21having a second light source and second transfer medium 22 fortransferring light from second optical transmitter 21.

Vehicle-mounted optical communication system 2 performs datacommunications at a high rate of 1 Gbps or higher.

Optical transmitter 11 is fixedly mounted in side mirrors 31 of vehicle3 and cameras 33 attached to front and rear portions of vehicle body 32of vehicle 3. The reference characters T in FIG. 6 represent tires ofvehicle 3.

Transfer medium 12 connects optical transmitters 11 in cameras 33 toswitching controller 34, and also connects switching controller 34 tofront monitor F.

Switching controller 34 selects connections between front monitor F andoptical transmitters 11 in cameras 33. Switching controller 34 comprisesan optical switch, for example.

Optical receiver 23 is incorporated in front monitor F. Optical receiver23 converts an optical signal into an electric signal to display imagescaptured by cameras 33 on front monitor F. In the present exemplaryembodiment, front monitor F and switching controller 34 are shown asbeing separate from each other. However, front monitor F and switchingcontroller 34 may be integral with each other.

According to the present exemplary embodiment, optical receiver 23includes photodetector 234 shown in FIG. 7 rather than photodetector 131according to the preceding exemplary embodiment. Optical receiver 23 isof the same structure as optical receiver 13 according to the precedingexemplary embodiment with respect to other details.

Photodetector 234 is capable of detecting light having a wavelength bandof 850 nm and light having a wavelength in the range from 1000 to 1100nm.

Photodetector 234 comprises n-type InP substrate 234A as a semiconductorsubstrate, optical absorption layer 234B disposed on InP substrate 234A,and cap layer 234C disposed on optical absorption layer 234B.

A buffer layer, a multiplier layer, an electric field relaxing layer,etc. may be interposed between optical absorption layer 234B and InPsubstrate 234A.

Optical absorption layer 234B comprises an InGaAs layer lattice-matchedto InP substrate 234A.

Cap layer 234C is made of a semiconductor material having a forbiddenbandwidth of 1.46 eV or greater, e.g., InAlAs. n-type electrode 234D isdisposed on cap layer 234C.

n-type electrode 234D is disposed on the reverse surface of in Psubstrate 234A.

Second optical transmitter 21 serves to transmit an optical signal inthe 850 nm band. According to the present exemplary embodiment, secondoptical transmitter 21 is incorporated in various devices including TVtuner 36, DVD device 37, and navigation device 38.

Though not shown, second optical transmitter 21 comprises a drivecircuit for receiving an electric signal generated by devices 36, 37, 38and a second light source device energized by the drive circuit.

The second light source device comprises surface-emitting laser 24 shownin FIG. 8.

Surface-emitting laser 24 comprises GaAs substrate 241 as asemiconductor substrate, first DBR (Distributed Bragg Reflector) layer242 disposed on GaAs substrate 241, lower cladding layer 243 disposed onfirst DBR layer 242, active layer 244 disposed on lower cladding layer243, upper cladding layer 245 disposed on active layer 244, currentconstricting layer 246 disposed on upper cladding layer 245, and secondDBR (Distributed Bragg Reflector) layer 247 disposed on currentconstricting layer 246.

First DBR layer 242 is an n-type semiconductor multilayer filmcomprising an alternate stack of Al0.1Ga0.9As and Al0.9Ga0.1As films.

Lower cladding layer 243 comprises an AlGaAs layer, for example.

Active layer 244 is an MQW (Multiple Quantum Well) layer comprising analternate stack of quantum well layers of GaAs and AlGaAs barrierlayers.

Upper cladding layer 245 comprises an AlGaAs layer, for example.

Current constricting layer 246 comprises an AlAs layer. Currentconstricting layer 246 includes low-resistance region 246A.Low-resistance region 246A is sandwiched between high-resistance regions(oxide regions) 246B having a higher resistance value thanlow-resistance region 246A and formed by a steam oxidation process.

Second DBR layer 247 is a p-type semiconductor multilayer filmcomprising an alternate stack of Al_(0.1)Ga_(0.9)As andAl_(0.9)Ga_(0.1)As films.

p-type electrode 248 is disposed on second DBR layer 247, and n-typeelectrode 249 is disposed on the reverse side of GaAs substrate 241.

Surface-emitting laser 24 has an oscillation wavelength band of 850 nm.Though not shown, surface-emitting laser 24 is housed in a packagefilled with an electrically insulative liquid or gel, as withsurface-emitting laser 15.

Vehicle-mounted optical communication system 2 will be described belowwith reference to FIG. 6 again.

Second transfer medium 22 comprises a ring-shaped optical fiber and alinear optical fiber connected to the ring-shaped optical fiber. Tosecond transfer medium 22, there are connected TV tuner 36, DVD device37, navigation device 38, as described above, and also monitor M, frontmonitor F, and second switching controller 35.

Second switching controller 35 selects connections between secondoptical transmitter 21 in the TV tuner, the DVD device, and thenavigation device, and monitor M or front monitor F. Second switchingcontroller 35 may comprise an optical switch.

Optical receiver 23 is also disposed in monitor M.

When camera 33 captures an image, it generates an electric signal basedon the captured image and sends the electric signal to opticaltransmitter 11 disposed in camera 33. In optical transmitter 11, drivecircuit 16 receives the electric signal and energizes surface-emittinglaser 15, Light emitted by surface-emitting laser 15 at a wavelengthranging from 1000 to 1100 nm is sent through transfer medium 12 toswitching controller 34. The light is sent through switching controller34 to front monitor F. In front monitor F, photodetector 236 of opticalreceiver 23 receives the optical signal and converts it into an electricsignal. The converted electric signal is decoded as it is processed byamplifying circuit 132 and code generating circuit 133. In this manner,the image is displayed on front monitor F.

When TV tuner 36 receives a digital satellite broadcast or the like, orDVD device 37 or navigation device 38 acquires an image, TV tuner 36,DVD device 37 or navigation device 38 generates electric signals such asa video signal, an audio signal, etc. These electric signals are sent tothe drive circuit of second optical transmitter 21, which energizessurface-emitting laser 24. Surface-emitting laser 24 emits light at awavelength band of 850 nm, and the optical signal is sent through secondtransfer medium 22 to second switching controller 35. Second switchingcontroller 35 determines a destination to which the optical signal is tobe transmitted, and the optical signal is transmitted to monitor M orfront monitor F.

In monitor M or front monitor F, photodetector 234 receives the opticalsignal and converts the optical signal into an electric signal. Theconverted electric signal is decoded as it is processed by amplifyingcircuit 132 and code generating circuit 133. In this manner, the imageis displayed on front monitor F or monitor M.

It was confirmed that vehicle-mounted optical communication system 2according to the present exemplary embodiment had a transmission rate of1 Gbps at an environmental temperature ranging from −40° C. to 125° C.

Vehicle-mounted optical communication system 2 was reliable for 5000hours at the environmental operating temperature of 100° C. ofsurface-emitting laser 15 (at this time, the environmental temperatureof surface-emitting laser 24 was 50° C.).

Vehicle-mounted optical communication system 2 has the same advantagesas with the first exemplary embodiment, and also offers the followingadvantages:

Information transmitted in the vehicle includes information, whosepromptness is important, with respect to the safety of the user in thevehicle and information, whose promptness is less important, for use inentertainment. If these types of information are transmitted in the samewavelength band, then, depending on the amount of information, thetransfer of the information with respect to the safety of the user maybe delayed.

According to the present exemplary embodiment, the information withrespect to the safety of the user is converted by surface-emitting laser15 into an optical signal at a wavelength ranging from 1000 nm to 1100nm, and the information for use in entertainment is converted bysurface-emitting laser 14 into an optical signal at a wavelength band of850 nm.

Since different wavelength bands are used depending on the promptness ofinformation, the transfer of the information with respect to the safetyof the user is prevented from being delayed.

According to the present exemplary embodiment, furthermore,photodetector 234 incorporated in front monitor F is capable ofdetecting both light at a wavelength band of 850 nm and light at awavelength ranging from 1000 nm to 1100 nm. Therefore, the spacerequired to install the photodetector is smaller than if photodetectorsfor detecting respective lights at those wavelengths are installed infront monitor F.

3rd Exemplary Embodiment

A third exemplary embodiment of the present invention will be describedbelow with reference to FIG. 9.

Vehicle-mounted optical communication system 4 according to the presentexemplary embodiment comprises optical transmitter 41 havingsurface-emitting laser 15 and surface-emitting laser 24 similar to thoseaccording to the second exemplary embodiment, transfer medium 12 fortransferring light from optical transmitter 41, and optical receiver 43for receiving the light transferred by transfer medium 12.

Optical transmitter 41 has a drive circuit similar to those according tothe previous exemplary embodiments. The drive circuit serves to energizesurface-emitting laser 15 and surface-emitting laser 24. Thesesurface-emitting lasers 15, 24 are accommodated in a package filled withan electrically insulative liquid or gel as with the above exemplaryembodiments.

Optical transmitter 41 is mounted in cameras 33, TV tuner 36, DVD device37, and navigation device 38. In each of the devices including thecameras, acquired image data are divided by a data divider, not shown,and the divided data are converted into packets with various ancillaryinformation (data name, source and destination addresses, transmissiontime, etc.) added as a transmission header to each of the data. Thepacketized data are send as an electric signal to the drive circuit ofoptical transmitter 41. The drive circuit energizes surface-emittinglaser 15 and surface-emitting laser 24. Surface-emitting laser 24generates an optical signal corresponding to the transmission header,and surface-emitting laser 15 generates an optical signal correspondingto the data itself. The optical signal corresponding to the transmissionheader and the optical signal corresponding to the data itself aretransferred in synchronism with each other. The optical signalcorresponding to the data itself can be transferred at a rate rangingfrom 1 Gbps to 5 Gbps.

Transfer medium 12 comprises a ring-shaped optical fiber and a linearoptical fiber connected to the ring-shaped optical fiber. To transfermedium 12, there are connected cameras 33, TV tuner 36, DVD device 37,navigation device 38, and controller 44.

Controller 44 serves to control destinations of optical signals fromdevices 33, 36, 37, 38, and determine which of monitor M and frontmonitor F optical signals from devices 33, 36, 37, 38 are to betransferred to.

Controller 44 includes optical receiver 43.

Optical receiver 43 comprises photodetector 434 shown in FIG. 10, and anamplifying circuit and a code generating circuit, not shown, similar tothose according to the above exemplary embodiments.

Photodetector 434 comprises n-type InP substrate 434A as a semiconductorsubstrate, optical absorption layer 434B disposed on InP substrate 434A,cap layer 434C disposed on optical absorption layer 434B, insulatinglayer 434D disposed on cap layer 434C, n-type InP layer 434E as asemiconductor layer disposed on insulating layer 434D, opticalabsorption layer 434F disposed on InP layer 434E, and cap layer 434Gdisposed on optical absorption layer 434F.

n-type electrode 434H is disposed on the reverse surface of InPsubstrate 434A.

Optical absorption layer 434B comprises an InGaAs layer lattice-matchedto InP substrate 434A.

Cap layer 434C is made of a semiconductor material having a forbiddenbandwidth of 1.46 eV or greater, e.g., p-type InP.

Insulating layer 434D comprises an Ru-doped InP layer, for example.

Optical absorption layer 434F is made of a semiconductor material havinga forbidden bandwidth greater than 1.15 eV, e.g., InAlGaAs.

Cap layer 434G is made of a semiconductor material having a forbiddenbandwidth greater than 1.49 eV, e.g., p-type InAlAs. p-type electrode434K is disposed on cap layer 434G.

Optical absorption layer 434B is disposed so as to fully coversubstantially the entire surface of InP substrate 434A, and cap layer434C is also disposed so as to fully cover substantially the entiresurface of optical absorption layer 434B.

Insulating layer 434D and InP layer 434E are smaller in planarconfiguration than cap layer 434C, and n-type electrode 434I is disposedon an area of cap layer 434C which is not covered with insulating layer434D and InP layer 434E.

Optical absorption layer 434F and cap layer 434G are smaller in planaris configuration than InP layer 434E, and n-type electrode 434J isdisposed on an area of InP layer 434E which is not covered with opticalabsorption layer 434F and cap layer 434G.

With photodetector 434 thus constructed, optical absorption layer 434Bis capable of absorbing light at a wavelength ranging from 1000 to 1100nm, and optical absorption layer 434F is capable of absorbing light at awavelength band of 850 nm.

An optical signal emitted from optical transmitter 41 includes anoptical signal at a wavelength band of 850 nm which corresponds to atransmission header and an optical signal at a wavelength ranging from1000 to 1100 nm which corresponds to data itself. Photodetector 434 canthus separately detect the optical signal at a wavelength band of 850 nmand the optical signal at a wavelength ranging from 1000 to 1100 nm. Theseparate optical signals are converted into respective electric signals,which are decoded as they are processed by the amplifying circuit andthe code generating circuit.

Controller 44 analyzes the electric signal corresponding to thetransmission header to determine which of monitor M and front monitor Fthe electric signal corresponding to the data itself is to betransferred to.

Front monitor F or monitor M displays an image based on the electricsignal sent from controller 44.

The present exemplary embodiment has the same advantages as with theabove exemplary embodiments, and also offers the following advantages:

According to the present exemplary embodiment, optical transmitter 41generates an optical signal at a wavelength band of 850 nm whichcorresponds to a transmission header and an optical signal at awavelength ranging from 1000 to 1100 nm which corresponds to dataitself. Since the transmission header is of a low volume and can betransmitted at a low rate, surface-emitting laser 24 is less liable tobe heated and is kept reliable.

The data itself is of a high volume and needs to be transmittedpromptly. Since surface-emitting laser 15, which is of high heatresistance, for generating an optical signal at a wavelength rangingfrom 1000 to 1100 nm is used to transmit the data itself,vehicle-mounted optical communication system 4 is kept reliable.

According to the present exemplary embodiment, photodetector 434comprises optical absorption layer 434F made of a semiconductor materialhaving a forbidden bandwidth greater than 1.15 eV and optical absorptionlayer 434B comprising an InGaAs layer. Therefore, photodetector 434 canseparately detect an optical signal at a wavelength band of 850 nm andan optical signal at a wavelength ranging from 1000 to 1100 nm.Consequently, single photodetector 434 can separate optical signals in aplurality of wavelength bands for decoding the optical signalsindividually.

The present invention is not limited to the above exemplary embodiments,but covers modifications, improvements, etc. insofar as they can achievethe object of the present invention.

For example, though surface-emitting lasers 15, 24 are accommodated inpackage 17 in the above exemplary embodiments, the present invention isnot limited to such a structure. Surface-emitting lasers 15, 24 may notbe accommodated in package 17. Not only surface-emitting lasers 15, 24but also photodetectors 131, 234, 434 of optical receivers 13, 23, 43may be accommodated in package 17 filled with an electrically insulativeliquid or gel. The electrically insulative liquid or gel in package 17reduces the coupling loss from 2 dB, which occurs if surface-emittinglasers 15, 24 and photodetectors 131, 234, 434 are not housed in package17, to 1 dB.

In the second exemplary embodiment, photodetector 234 including singleoptical absorption layer 234B for detecting an optical signal at awavelength band of 850 nm and an optical signal at a wavelength rangingfrom 1000 to 1100 nm is employed. However, photodetector 434 accordingto the third exemplary embodiment may be employed.

In the third exemplary embodiment, after controller 44 converts anoptical signal into an electric signal, controller 44 sends the electricsignal to monitor M or front monitor F. Alternatively, controller 44 maysend an optical signal to monitor M or front monitor F.

Specifically, the controller separates an optical signal sent fromoptical transmitter 41 into an optical signal corresponding to atransmission header and an optical signal corresponding to data itself.The controller converts the optical signal corresponding to atransmission header into an electric signal with a photodetector.Thereafter, the controller sends the optical signal corresponding todata itself to the monitor or the front monitor based on the electricsignal corresponding to a transmission header. Unlike the thirdexemplary embodiment, the controller may have only a photodetector fordetecting an optical signal corresponding to a transmission header (anoptical signal at a wavelength band of 850 nm). The monitor or the frontmonitor may have only a photodetector for detecting an optical signalcorresponding to data itself (an optical signal at a wavelength in therange from 1000 to 1100 nm).

Since data can be sent as an optical signal between the controller andmonitor M or between the controller and front monitor F, thetransmission rate for data between the controller and monitor M orbetween the controller and front monitor F is increased.

In the second exemplary embodiment and the third exemplary embodiment,TV tuner 36, DVD device 37, and navigation device 38 incorporate opticaltransmitters. However, TV tuner 36, DVD device 37, and navigation device38 incorporate optical receivers.

For example, the monitors may incorporate optical transmitters and TVtuner 36, DVD device 37, and navigation device 38 may incorporateoptical receivers for allowing signals to be transferredbidirectionally.

1. A vehicle-mounted optical communication system, adapted to be mountedon a vehicle, for performing data transmission with optical signals,comprising: a first optical transmitter adapted to be mounted on thevehicle and including a first surface-emitting laser, for transmittingan optical signal generated by said first surface-emitting laser, saidfirst surface-emitting laser including an active layer of a multiplequantum well structure having a quantum well layer of In_(x)Ga_(1-x)As(0.15≦x≦0.35), said first surface-emitting laser having an oscillationwavelength ranging from 1000 nm to 1100 nm inclusive; and an opticalreceiver adapted to be mounted on the vehicle and connected to saidfirst optical transmitter by a first optical transfer path, forreceiving said optical signal transmitted from said first opticaltransmitter through said first optical transfer path.
 2. Avehicle-mounted optical communication system according to claim 1,wherein said first optical transmitter further includes a secondsurface-emitting laser having an oscillation wavelength band of 850 nm,for transmitting an optical signal generated by said secondsurface-emitting laser.
 3. A vehicle-mounted optical communicationsystem according to claim 2, wherein said first surface-emitting lasergenerates an optical signal representing data itself and said secondsurface-emitting laser generates an optical signal representing atransmission header.
 4. A vehicle-mounted optical communication systemaccording to claim 1, further comprising: a second optical transmitterincluding a second surface-emitting laser having an oscillationwavelength band of 850 nm, for transmitting an optical signal generatedby said second surface-emitting laser; wherein said optical receiver isconnected to said second optical transmitter by a second opticaltransfer path, for receiving said optical signal transmitted from saidsecond optical transmitter through said second optical transfer path. 5.A vehicle-mounted optical communication system according to claim 4,wherein said second optical transmitter is used to transmit informationwhich is less prompt than information to be transmitted by said firstoptical transmitter.
 6. A vehicle-mounted optical communication systemaccording to claim 2, wherein said optical receiver includes aphotodetector comprising an optical absorption layer which comprises anInGaAs layer disposed on a semiconductor substrate and a cap layerdisposed on said optical absorption layer and having a forbiddenbandwidth of 1.46 eV or greater, and said optical receiver receives boththe optical signal generated by said first surface-emitting laser andthe optical signal generated by said second surface-emitting laser.
 7. Avehicle-mounted optical communication system according to claim 2,wherein said optical receiver includes a photodetector comprising afirst optical absorption layer which comprises an InGaAs layer disposedon a semiconductor substrate, a first cap layer disposed on said opticalabsorption layer and having a forbidden bandwidth of 1.46 eV or greater,an insulating layer disposed on said cap layer, a semiconductor layerdisposed on said insulating layer, a second optical absorption layerdisposed on said semiconductor layer and having a forbidden bandwidth of1.15 eV or greater, and a second cap layer disposed on said secondoptical absorption layer and having a forbidden bandwidth of 1.46 eV orgreater, and said optical receiver receives both the optical signalgenerated by said first surface-emitting laser and the optical signalgenerated by said second surface-emitting laser.
 8. A vehicle-mountedoptical communication system according to claim 1, wherein at least saidfirst surface-emitting laser is housed in a sealed package which isfilled with an electrically insulative liquid or gel.
 9. Avehicle-mounted optical transmitter for use in an optical communicationsystem, adapted to be mounted on a vehicle, for performing datatransmission with optical signals, comprising: a first light sourcedevice including a first surface-emitting laser, for transmitting anoptical signal generated by said first surface-emitting laser, saidfirst surface-emitting laser including an active layer of a multiplequantum well structure having a quantum well layer of In_(x)Ga_(1-x)As(0.15≦x≦0.35), said first surface-emitting laser having an oscillationwavelength ranging from 1000 nm to 1100 nm inclusive; and a drivecircuit for energizing said first surface-emitting laser of said firstlight source device based on an electric signal.
 10. A vehicle-mountedoptical transmitter according to claim 9, further comprising a secondlight source device including a second surface-emitting laser having anoscillation wavelength band of 850 nm, for transmitting an opticalsignal generated by said second surface-emitting laser.
 11. Avehicle-mounted optical transmitter according to claim 10, wherein saidfirst surface-emitting laser generates an optical signal representingdata itself and said second surface-emitting laser generates an opticalsignal representing a transmission header.
 12. A vehicle-mounted opticaltransmitter according to claim 9, wherein at least said firstsurface-emitting laser is housed in a sealed package which is filledwith an electrically insulative liquid or gel.
 13. A vehicle-mountedoptical receiver for use in an optical communication system, adapted tobe mounted on a vehicle, for performing data transmission with opticalsignals, comprising: a photodetector for detecting both a first opticalsignal at a wavelength band of 850 nm and a second optical signal at awavelength ranging from 1000 nm to 1100 inclusive and converting thefirst and second optical signals into electric signals, saidphotodetector including an optical absorption layer which comprises anInGaAs layer disposed on a semiconductor substrate and a cap layerdisposed on said optical absorption layer and having a forbiddenbandwidth of 1.46 eV or greater; and an amplifying circuit for receivingsaid electric signals from said photodetector and amplifying thereceived electric signals.
 14. A vehicle-mounted optical receiver foruse in an optical communication system, adapted to be mounted on avehicle, for performing data transmission with optical signals,comprising: a photodetector for detecting both a first optical signal ata wavelength band of 850 nm and a second optical signal at a wavelengthranging from 1000 nm to 1100 inclusive and converting the first andsecond optical signals into electric signals, said photodetectorincluding a first optical absorption layer which comprises an InGaAslayer disposed on a semiconductor substrate, a first cap layer disposedon said optical absorption layer and having a forbidden bandwidth of1.46 eV or greater, an insulating layer disposed on said cap layer, asemiconductor layer disposed on said insulating layer, a second opticalabsorption layer disposed on said semiconductor layer and having aforbidden bandwidth of 1.15 eV or greater, and a second cap layerdisposed on said second optical absorption layer and having a forbiddenbandwidth of 1.46 eV or greater; and an amplifying circuit for receivingsaid electric signals from said photodetector and amplifying thereceived electric signals.
 15. A vehicle-mounted optical communicationsystem according to claim 4, wherein said optical receiver includes aphotodetector comprising an optical absorption layer which comprises anin GaAs layer disposed on a semiconductor substrate and a cap layerdisposed on said optical absorption layer and having a forbiddenbandwidth of 1.46 eV or greater, and said optical receiver receives boththe optical signal generated by said first surface-emitting laser andthe optical signal generated by said second surface-emitting laser. 16.A vehicle-mounted optical communication system according to claim 4,wherein said optical receiver includes a photodetector comprising afirst optical absorption layer which comprises an InGaAs layer disposedon a semiconductor substrate, a first cap layer disposed on said opticalabsorption layer and having a forbidden bandwidth of 1.46 eV or greater,an insulating layer disposed on said cap layer, a semiconductor layerdisposed on said insulating layer, a second optical absorption layerdisposed on said semiconductor layer and having a forbidden bandwidth of1.15 eV or greater, and a second cap layer disposed on said secondoptical absorption layer and having a forbidden bandwidth of 1.46 eV orgreater, and said optical receiver receives both the optical signalgenerated by said first surface-emitting laser and the optical signalgenerated by said second surface-emitting laser.